Signal mixers



pt- 6, 1966 M. J. BOCK 3,271,686

SIGNAL MIXERS Filed Oct. 8, 1962 2 Sheets-Sheet 1 UTILIZATI RF DEVICEON GENERATOR LOCAL OSCILLATOR LOCAL OSCILLATOR From RF GenerutorlG lNVE/VTOR FIG. 3 MARVIN 00K AGENT Sept. 6, 1966 Filed Oct. 8, 1962 2 Sheets-Sheet 2 FIG. 4

2 5 LOW 8-2% a qb HIGH RF OUTPUT- PASS PASS S|GNAL FILTER El E f FILTER INPUT LOCAL OSC/LLATOR /NVEN7'0/? fl/WARV/N 506K E) f 0) pm AGE/VT United States Patent 3,271,686 SIGNAL MIXERS Marvin J. Bock, Santa Barbara, Calif, assignor to Raytheon Company, Lexington, Mass, a corporation of Delaware Filed Oct. 8, 1962, Ser. No. 229,047 Claims. (Cl. 325-445) This invention relates generally to signal mixers and more particularly to a balanced mixer employing crystal diodes as the mixing elements.

Superheterodyne receivers such as employed in radar systems typically have an intermediate frequency located in the range 30120 megacycles, and the bandwidth of such receivers is generally between five and twenty megacycles. In some applications higher intermediate frequencies are preferable; however, opera-tion at these higher frequencies is limited by the transmission capacity of the waveguide transmission lines. Furthermore, in some applications it is preferable to operate over an intermediate frequency bandwidth ranging as Wide as from 5 to 2000 megacycles. Heretofore, crystal mixers have included a section of waveguide loaded with one or more crystal diodes, one end of each crystal being equipped with a coaxial output coupling. In operation, the two signals to be mixed are fed in from opposite ends of the waveguide, and the sum or difference frequency is coupled from the coaxial output. Other crystal mixer structures include one or more crystal diodes loaded in a waveguide separated by a conductive septum providing an RF short between the crystals. Both signals to be mixed are fed from each end of the waveguide, one signal in opposite phase, and the difference or sum frequency is obtained from a coaxial connector coupled to both of the crystals.

The operating frequency and bandwidth of crystal mixers constructed as described above are limited by the dimensions of the waveguide into which the crystals are loaded. The frequency is limited at the low end by the cutoff frequency of the waveguide and at the high end by the mode of propagation of the signals which are mixed. The bandwidth of output from the coaxial connector is limited by impedance which must be matched at this output.

It is one object of the present invention to provide a novel type of mixer in which the bandwidth and operating frequency limitations described above are avoided.

It is another object to provide a wide band mixer in which the transmission lines for the signals which are mixed are substantially uncoupled from each other so that neither signal will couple into the transmission line provided for the other signal.

The present invention includes two separate transmission line structures, one which is of broader band for conducting the highest frequency such as RF, and another which may be of more narrow band for conducting the lower frequency such as local oscillator frequency. The wave paths provided by each of these structures cross at a common point in such a manner that neither of the signals to be mixed will be conducted by the transmission line provided for the other signal. Mixing is accomplished in, for example, a pair of crystal diodes which are preferably oriented to couple the output or IF into the transmission line provided for the IF signal, and a filter is provided at the output of this transmission structure to reflect RF in proper phase While passing the IF substantially unattenuated.

Specific embodiments of the invention include a multiple element transmission line such as a coaxial line for conducting the RF in a TEM mode and a waveguide for conducting the local oscillator signal joined together so that one element of the coaxial line passes through the waveguide and is insulated from the walls of the 3,271,686 Patented Sept. 6, 1966 waveguide, while the other element of the coaxial line connects to the walls of the waveguide. A pair of crystal diodes are loaded in the waveguide on opposite sides of the center conductor, and electrically oriented in opposite direction with respect to the center conductor so that IF couples into the coaxial line. This electrical orientation is preferred, although it is possible to operate the device with both crystal diodes electrically oriented the same with respect to the center conductor. This, however, would couple the IF into the waveguide rather than into the coaxial line.

Other features and objects of the invention will be apparent from the following specific description of embodiments taken in conjunction with the figures in which:

FIG. 1 illustrates a front-sectional view of a crystal mixer including features of the invention;

FIG. 2 illustrates a side-sectional view of the mixer;

FIG. 3 illustrates a three-quarter isometric view of the mixer; and

FIG. 4 is a sketch illustrating the orientation of electric vectors of the RF, local oscillator, and IF to aid in understanding operation of the device.

FIGS. 1-4 illustrate an embodiment of the invention including crystal diodes for mixing two signals which, for convenience, will be hereinafter referred to as RF signal and local oscillator signal to produce at least one-third signal referred to as IF signal. As shown in FIG. 3, a multiple element transmission line such as coaxial line 1 and a waveguide transmission line 2 are joined. As illustrated, these two transmission lines are joined with axes at substantially right angles. The outer conductor 3 of the coaxial line is attached to the walls of the waveguide, while the center conductor 4 of the coaxial line is electrically insulated from the walls of the waveguide. More particularly, the coaxial transmission line is formed in two sections 5 and 6 denoted RF and IF sections and joined through the waveguide by a section of strip transmission line 7. The outer conductor of each section is attached to each of the narrow walls of the waveguide and the center conductor passes through openings in these walls. Crystal diodes 8 and 9 are loaded in the waveguide and electrically directed transverse to the axes of the transmission lines. The diodes are preferably located on opposite sides of the center conductor 11 of the section of strip transmission line. Center conductor 11 is located within the walls of the waveguide and is preferably shaped as a flat plate so that it forms a short section of three-element strip transmission line 7 in conjunction with the broad walls of the waveguide.

The crystal diodes 8 and 9 are preferably electrically directed so that the IF or frequency generated within the crystals couples to the short section of strip transmission line 7 and from there to the coaxial line 6 rather than coupling into the waveguide. Accordingly, the crystals are electrically directed as illustrated in FIGS. 1 and 2 and each has one terminal connected to the conductor 11. This preferred electrical orientation is achieved by employing two crystals designed with opposite electrical directions; that is, one directed toward its base, and the other directed away from its base so that the base of each of the crystals can be mounted within insulating sleeves such as 12 and 13 which are embedded in the Wide walls of the waveguide, while the pins of each of the crystals are in direct contact with the center conductor 11.

The center conductor 11, as already mentioned, is preferably substantially flat and is connected at its opposite ends to the center conductor of the coaxial line.

sections 5 and 6. The impedance match between the section of strip transmission line and the two sections of coaxial line is improved somewhat by tapering the ends of the center conductor 11 where they join the center conductors of the coaxial lines. The central portion of the conductor 11, however, is preferably wider than the diameter of the center conductor of the coaxial lines as required to provide optimal impedance match.

A high pass filter 14 is provided in the RF section of the coaxial line, and a low pass filter 15 is provided in the IF section. The high pass filter passes RF but reflects IF in the proper phase while the low pass filter passes IF but reflects RF, in the proper phase thus providing isolation between the RF generator 16 and a utilization device 17.

In operation, the bases of the crystal diodes 8 and 9 are preferably coupled to ground through resistors 18 and 19, respectively. These resistors are adjusted to provide substantially the same impedances in forward directions between the pins of each of the crystals which are connected to the center conductor 11 and ground, thus insuring a balance of IF signal amplitude generated within each crystal. RF signals from, for example, an antenna or a generator 16 are conducted by the RF coaxial section 5 through the high pass filter 14 and into the waveguide, while local oscillator signal is introduced into the waveguide from a local oscillator generator 21 coupled to the waveguide by a probe 22 as shown in FIG. 2. Thus, the RF and local oscillator signals both propagate through the guide by a probe 22 as shown in FIG. 2. Thus, the RF and local oscillator signals both propagate through the crystal diodes 8 and 9. FIG. 4 illustrates the relative orientation of electric fields of the RF, local oscillator and IF frequencies propagating within the waveguide 2 in the immediate vicinity of the crystals. The arrows denoted E' represent the electric fields of RF which is conducted substantially in a TEM mode along the short section of strip transmission line 7 formed by center conductor 11 and the broad walls of the waveguide. The arrow E represents the electric vector of the local oscillator signal which is conducted in, for example, a TE mode in the waveguide. This vector may be considered as two vectors E and B both directed in the same direction, one running from the bottom wall of the guide to the center conductor 11, and the other running from the center conductor 11 to the top wall of the guide. Thus, the fields of local oscillator signal propagating through each of the crystal diodes 8 and 9 are in phase in each of the diodes, while the fields of IF signal propagating through the diodes are out of phase in each. As a result, the fields of IF signal generated within each of the diodes is out of phase as indicated by the arrows E which both extend away from the center conductor 11 toward opposite walls of the waveguide. This electric field orientation of the IF signal defines a substantially TEM propagating mode, and, therefore, the IF signal cannot couple into the waveguide, but it can couple into the coaxial line. The IF will, therefore, couple out through the low pass filter 15 for application to a utilization device 17, but will not couple through the high pass filter 14.

If the electrical directions of the crystals 8 and 9 were such that they were both directed either toward or away from the central conductor 11, then both the upper and lower components of electric field of the IF signal would be in the same direction, and the IF signal would couple into the waveguide rather than the coaxial line. It is preferred that the IF signal couple into the coaxial line because it is broader hand than the waveguide permitting broader band operation of the mixer.

This completes the description of a crystal mixer incorporating features of the invention in which transmission lines conducting separate signals cross and frequency mixing devices such as crystal diodes are located at the point of crossing to produce a sideband frequency which is conducted by only one of the transmission paths, the

modes of propagation of the mixed signals being such that neither will couple into the transmission path provided for the other signal, thereby providing a degree of isolation between the two mixed signals. Other mixing devices could be substituted for the crystal diodes; for example, mixing could be accomplished with a triode or pentode or with any substantially unidirectionally conductive device having a substantially nonlinear range of operation without deviating from the scope of the invention vas set forth in the following claims,

What is claimed is:

1. An electrical device comprising in combination, an elongated waveguide, a three-element transmission line comprising first and second coaxial conducting elements including center and outer conductors with said outer conductors mounted upon and extending outwardly from opposite sides of the waveguide, and a third element extending through the interior of the waveguide with its opposed ends connected respectively to the center conductors of said first and second elements, said first element comprising means for transmitting a first signal to the third element, the waveguide comprising means for transmitting a second signal to said third element, and mixing means at said third element for mixing said first and second signals, producing a resultant third signal, and coupling said third signal to only said second element of the threeelement transmission line.

2. A device as set forth in claim 1 wherein said waveguide is rectangular in horizontal cross-section and the outer conductors of said first and second elements extend transversely from the narrow sidewalls of the waveguide.

3. A device as set forht in claim 1 wherein said third element defines with the broad walls of said waveguide a strip transmission line.

4. A device as set forth in claim 1 wherein said mixing means comprises a pair of crystal diodes disposed within the waveguide in the paths of said first and second signals and separated by the third element which provides a short between the diodes.

5. An electrical device comprising in combination, an elongated rectangular waveguide, a three-element transmission line comprising first and second coaxial conducting elements including center and outer conductors with said outer conductors mounted upon and extending outwardly from opposite sides of the waveguide, and a strip transmission line defined by a center member and the opposing broad walls of said waveguide with the opposed ends of said center member connected respectively to the center conductors of said first and second coaxial elements, said first coaxial element comprising means for transmitting a first signal to the strip transmission line, the waveguide comprising means for transmitting a second signal to said strip transmission line, and mixing means at said strip transmission line for mixing said first and second signals, producing a resultant third signal, and coupling said third signal to only said second element of the transmission line, said mixing means comprising a pair of crystal diodes disposed within the waveguide in the paths of said first and second signals and separated and shorted by the strip transmission line.

References Cited by the Examiner UNITED STATES PATENTS 2,514,678 7/1950 Southworth 33254 2,950,384 8/1960 Hines 325-445 3,066,290 11/1962 Whitehorn 325-446 X KATHLEEN H. CLAFFY, Primary Examiner.

DAVID G. REDINBAUGH, Examiner.

R. F. ROTELLA, R. S. BELL, Assistant Examiners. 

1. AN ELECTRICAL DEVICE COMPRISING IN COMBINATION, AN ELONGATED WAVEGUIDE, A THREE-ELEMENT TRANSMISSION LINE COMPRISING FIRST AND SECOND COAXIAL CONDUCTING ELEMENTS INCLUDING CENTER AND OUTER CONDUCTORS WITH SAID OUTER CONDUCTORS MOUNTED UPON AND EXTENDING OUTWARDLY FROM OPPOSITE SIDES OF THE WAVEGUIDE, AND A THIRD ELEMENT EXTENDING THROUGH THE INTERIOR OF THE WAVEGUIDE WITH ITS OPPOSED ENDS CONNECTED RESPECTIVELY TO THE CENTER CONDUCTORS OF SAID FIRST AND SECOND ELEMENTS, SAID FIRST ELEMENT COMPRISING MEANS FOR TRANSMITTING A FIRST SIGNAL TO THE THIRD ELEMENT, THE WAVEGUIDE COMPRISING MEANS FOR TRANSMITTING A SECOND SIGNAL TO SAID THIRD ELEMENT, AND MIXING MEANS AT SAID THIRD ELEMENT FOR MIXING SAID FIRST AND SECOND SIGNALS, PRODUCING A RESULTANT THIRD SIGNAL, AND COUPLING SAID THIRD SIGNAL TO ONLY SAID SECOND ELEMENT OF THE THREEELEMENT TRANSMISSION LINE. 